It is well known that the aromatic polyester copolymers composed of the terephthalic acid, isophthalic acid and bisphenol A components find use in a wide range of areas in view of their superior properties such as their excellent mechanical and thermal properties, resistance to chemicals and fire retardancy. As methods for producing these aromatic polyester copolymers, there can be mentioned (1) the interfacial polycondensation process wherein the reaction is carried out in a reaction medium consisting of water and a water-immiscible solvent in the presence of a basic inorganic compound, (2) the solution polycondensation process wherein the reaction is carried out in an organic solvent in the presence of a tertiary amine, or (3) the melt polycondensation process wherein the reaction is performed using a high vacuum at elevated temperatures. Of these processes for producing the aromatic polyester copolymers, it can be said that the employment of the interfacial polycondensation process is to be preferred in view of such advantages as (a) the reaction can be carried out at room temperature and atmospheric pressure and be completed in a short period of time with no side reactions, (b) the production of the copolymer can be achieved by the use of a simple reaction apparatus, and (c) since there is a lesser inclusion of a catalyst, reaction solvent or dehydrochlorinating agent, a less colored polymer can be obtained. On the other hand, there are a number of defects that must be improved, such as (d) difficulty is experienced in separating the aqueous phase and water-immiscible solvent phase at the time of washing the reaction product mixture, with the consequence that there is a need for a complicated processing step, and (e) the step of solidification in which the intended polymer is recovered from the water-immiscible solvent phase cannot necessarily be carried out fully efficiently.